<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>G-Flex</title>
    <style>
        .demo{
            display: flex;
        }
        .child-1{
            flex:auto;
            order:2;
            background: #000;
        }
        .child-2{
            flex:auto;
            order:1;
            background: rgb(255, 2, 2);
        }

    </style>
</head>
<body>

<div class="demo">
    <div class="child-1">
        1
    </div>
    <div class="child-2">
        2
    </div>
</div>

</body>
</html>